(a.) Field of the Invention
The present invention relates generally to static structures. More specifically, it relates to concrete panel structures in a form which is useful for trusses or for use in bridge decks. The present invention also relates to methods of bridge construction and to methods of producing deck panels for use in bridge structures.
(b.) Description of the Prior Art
Typically, traffic bearing bridges are constructed using concrete bridge deck panels supported by a specifically designed substructure. Such concrete panels are normally supported at their longitudinal edges by at least a pair of separated support members, such as beams, which beams extend longitudinally in the same direction as what is defined herein as the length of the panels. State-of-the-art concrete bridge deck panel construction has traditionally been comprised of a slab constructed of one or more layers of concrete having a flexural reinforcing structure distributed throughout the concrete layer. Such a flexural reinforcing structure is generally in the form of a matrix of overlapping steel re-enforcing bars (re-bars) or steel strands, which are spaced from both the upper surface and the lower surface of the concrete panel. In accordance with traditional practice, this flexural reinforcing structure is included in the concrete for the purpose of carrying bending moment tension stresses which are placed on the concrete panel due to loading and unloading of the top surface, for example, by the passage of vehicles on or adjacent to the top surface.
It has traditionally been believed that structural flexural reinforcing material such as steel reinforcing bars (re-bars), are required throughout the concrete of such a panel, and especially in groups in the top and bottom halves of the panel near both the top and bottom surfaces of the panel. In the current state-of-the-art, it is believed to be necessary to use both top and bottom structural flexural reinforcing material re-bars in order to restrain cracking of the top surface and of the bottom surface due to applied loads.
The lower group of flexural reinforcing material in the bottom half of the panel normally consists of a first plurality of re-bars which form a layer. This first plurality of re-bars are transverse to both the length dimension of the panel and to the load-carrying beams which will support the panel. For structural purposes, this lower layer of transverse flexural re-bars material carries the positive moment tensile stresses which are applied to the panel. A second lower layer of flexural reinforcing material, consisting of a second plurality of re-bars which are parallel to both the length dimension of the panel and to the load-carrying, support beams (and transverse to the first lower layer of re-bars) is located directly above the first lower layer of re-bars. For structural purposes, this second lower layer of flexural reinforcing material re-bars distributes the bending moment loads which are applied to the panel longitudinally. Both lower layers of flexural reinforcing material re-bars provide control of temperature shrinkage cracking at the lower surface of the panel. Under current codes, for most beam spacings which are up to about eleven feet apart, the longitudinal bottom group of flexural reinforcing material constitutes about one-half to about two-thirds of the main reinforcement of the panel. The two lower layers of flexural reinforcing material are usually joined together to form a mat or matrix.
Further, in accordance with current practice, another group of main flexural reinforcing material is located in the top half of the panel near the upper surface of the concrete panel. It consists of a first upper layer comprised of a plurality of flexural reinforcing materials, which are designed to carry the negative moment tensile stresses which are applied to the panel, and a second lower layer comprised of a plurality of flexural reinforcing materials, which are designed to hold the uppermost flexural reinforcing materials in position during concrete placement. Both upper layers of flexural reinforcing material re-bars are intended to provide control of temperature shrinkage cracking at the upper surface of the panel. The upper group of flexural reinforcing materials is also usually in the form of a mat or matrix, which matrix is sized and oriented substantially identically to and also parallel to the flexural reinforcing matrix group in the lower half of the panel.
The flexural reinforcing material composed of steel re-bars which are not coated or connected to a sacrificial anode corrode readily when exposed to thawing salts and other corrosive elements, and even to ordinary water.
Despite the above described traditional flexural reinforcing of concrete bridge deck panel structures, concrete bridge deck panels have been found to deteriorate rapidly and to require costly rehabilitation or replacement from time-to-time. It has been recently estimated, for example, that the use of thawing salts on bridges in the United States causes $1.6 billion dollars worth of damage annually. Similar problems exist outside of the United States. Thus, there is a world-wide need to reduce the deterioration of concrete bridge deck panels without reducing the ability of the bridge deck panels to resist moment stresses imposed thereon by traffic loads.
It has been determined that much of the deterioration of concrete bridge deck panels is actually attributable to the corrosion of the traditional flexural reinforcing steel re-bars in the upper half of such bridge deck panels. It had been the common practice, until the late 1960's, to construct most concrete bridge deck panels over girder bridges with the bottom flexural reinforcing bars bent up over the supporting elements, such as beams or girders. Because of their shape, such bent up flexural strength reinforcing bars are sometimes referred to as "crank bars," because they resemble crankshafts. In the late 1960's the use of thawing salts on roads became quite prevalent. Subsequently the use of continuous straight flexural reinforcing top bars, or re-bars replaced the use of crank bars, because it was found to be more cost efficient to use more flexural reinforcing bars than to bend and place crank bars. As a result, this practice substantially increased the amount of corrodible steel re-bar material in the top of the deck panel. Bridge deck panels of this era were also constructed with only about 1.5 inches (3.8 cm) of protective concrete cover over the continuous straight top bars or re-bars.
During the early 1970's, the protective concrete cover over the top re-bars was generally increased to greater than about 2 inches (5.1 cm). At the same time, construction practices were improved so that reduction of the thickness of the top cover during panel placement, was avoided. It was believed that the additional thickness of the top cover would limit or slow cracking of the top surface, and thus lengthen the time that it took for chlorides from thawing salts and other corrosive elements to penetrate to the level of the re-bars contained in the upper portion of the concrete panel.
The understanding that chlorides from thawing salts and other corrosive materials corrode the re-bars in the upper half of the concrete panel and thus constitute the source of significant cracking and deterioration of the top surface of the bridge deck panel is important to the present invention.
Surprisingly, the additional thickness of concrete top cover included in bridge deck panel designs during the 1970's did not extend bridge deck panel life significantly. Subsequently, in most jurisdictions in which thawing salt is used, it became the practice to take steps to make bridge deck panels more impervious to the penetration of moisture, salt and other corrosive materials. It was believed that if the salt and other corrosive materials could not reach the re-bars in the upper half of the concrete layer, that the corrosion problem would be solved. Consequently, richer concrete mixes which were known to be more impervious to salts than traditional concrete mixes were utilized, and as a result the use of concrete having greater load bearing strengths then became standard practice. However, the use of richer concrete mixes led to yet another problem, in that such concrete exhibited increased temperature change shrinkage characteristics.
It is believed that the increased temperature shrinkage change of the richer concrete mixes may be responsible for additional cracks developing in the top surface of the concrete in recently constructed deck panel structures. Of course, such cracks will allow thawing salts and other corrosive materials to reach the corrodible re-bars in the upper half of the concrete panel and cause them to corrode, and thereby cause deterioration of the panel.
It is also known that cracking in the upper surface of concrete bridge deck panels can be avoided by careful control of the concrete mix and by concrete placement techniques. However, to be successful, such a strategy requires careful selection and proportioning of materials, and meticulous concrete placement and curing practice. These techniques have not been widely employed as part of a bridge deck construction strategy because it was thought that control of negative moment stresses in the upper surface of bridge decks was the dominate requirement for the restraint of cracking in the upper surface.
Several barrier technologies have been developed to stop or limit corrosion of flexural reinforcing re-bar materials which are located in the top half of concrete bridge deck panels from contact with thawing salts and other corrosive materials. Such barrier technologies include, for example, surface membranes, dense concrete, latex modified concrete, epoxy coated re-bars and the like. These barrier systems have had only moderate success.
Epoxy coated re-bars have proven to provide the most satisfactory corrosion protection, since such coatings, if continuous, virtually eliminate all actual contact between the re-bars and the thawing salts or other corrosive materials. However, it will be recalled that such re-bars are normally installed as matrices, which are often connected by tie wires and chains to the re-bar matrix in the lower portion of the concrete. The connecting tie wires and chains are usually electrically conductive. It has been found that placing a matrix of epoxy coated re-bars in the upper half of the concrete panel into electrical connection with the uncoated matrix of re-bars in the lower half of the panel allows an electrical half-cell to develop. The existence of such a half-cell encourages corrosion of the upper matrix of epoxy coated flexural reinforcing material. Additionally, epoxy coating re-bars apparently do not bond with the concrete in the panel as well as uncoated re-bars. Therefore, when epoxy coated re-bars are used in the top half of a concrete panel, once surface cracking is initiated, the length and width of cracks in the top surface tend to be larger than they would be had uncoated re-bar been used.
Waterproofing membrane barrier systems have been coated on the top surface of concrete panels. One potential problem with such waterproofing membrane barrier systems is that, should any moisture manage to migrate or collect below the membrane, it creates a severe environment in which corrosion can occur, whether or not salts or other corrosive materials are present. Furthermore, such barrier systems may conceal the deterioration of the top of the concrete from view, thereby delaying remedial maintenance until deterioration has become quite severe.
The above sequence of developments in the prior art of concrete bridge deck panels has been extremely costly. The combined effects of the additional thickness of the concrete, the use of epoxy coated re-bars in the upper portion of the bridge deck panel, the coating of waterproofing membrane systems on the top surface, and the increased girder weight necessary to carry the greater dead load of thicker deck panels, have all increased the cost of bridge deck panel systems by perhaps as much as 30-50%. Furthermore, despite the recognition of the problems caused by the corrosion of upper half flexural reinforcing re-bar, and the various technologies which have been developed to combat them, and even with the increased cost, deterioration of bridge deck panels still is a problem which has not been satisfactorily resolved.
Recently, a great deal of research has been conducted in an effort to develop means to protect the flexural reinforcing bar matrix in the top half of the panels from the effects of corrosion. The effectiveness of these efforts has been reported in National Cooperative Highway Research Program Report #297 (NCHRP 297), Evaluation of Bridge Deck Protective Strategies, September, 1987.
In the other known prior art, Mingolla U.S. Pat. No. 4,271,555 and Barnoff U.S. Pat. No. 4,604,841 are both examples of bridge deck panel structures which attempt to overcome certain problems of construction. However, while there are certain novel features to these particular deck panel constructions, both of them use conventional flexural reinforcing steel bar materials near both the upper as well as the lower surface of the deck panel structure.
Recent known patents which have been awarded for bridge deck protection systems, include Jacobs U.S. Pat. No. 4,151,025; U.S. Pat. No. 4,708,888; and Marzocchi U.S. Pat. No. 4,319,854. They teach, respectively, a membrane barrier system, an electro-chemical "cathodic protection" system, and a combination membrane and electro-chemical system.
Through various research efforts, it has been found that transverse cracking generally occurs at the top surface of the panel substantially directly over the layer of transverse flexural reinforcing bars which are in the top half of a bridge deck panel. Such cracks are a significant factor in the deterioration of bridge deck panels, since, as already noted, they allow salts, other corrosive elements, and water to reach the flexural reinforcing bars which are in the top half of the panel and cause them to corrode, thereby accelerating deterioration of the panel. Surprisingly, these cracks form at about right angles to the direction that they would be expected to form if they were due to the stresses caused by the predicted bending moments to which the panel is subjected. However, it is now noted that the observed crack patterns are consistent with tensile stresses due to concrete shrinkage and the effects of temperature changes. This indicates that the control of the formation of transverse cracks directly over the top transverse reinforcing bars due to concrete shrinkage and temperature changes at the surface of bridge deck panels is of paramount importance in avoiding deck panel deterioration. However, effective means for its avoidance are not known to have been previously proposed.
It is well known that the use of either fibers or fabric serves to effectively control upper surface cracking due to volume changes from temperature and shrinkage. Such reinforcement materials can be used, in at least the concrete which forms the uppermost portion of a bridge deck panel, to control surface cracking caused by temperature shrinkage changes does not require careful control of the concrete mix, nor careful placement of the concrete in order to be successful. Romauldi U.S. Pat. No. 3,429,094 and Kobayashi U.S. Pat. No. 4,565,840 teach the use of fiber reinforcement materials for crack control in concrete. The use of various fiber materials for reinforcement concrete is discussed in the Manual of Concrete Practice, ACI. The use of fiber reinforcement materials to restrain cracking due to changes from temperature shrinkage has now become more common then the well established practice of using steel welded wire fabric reinforcement materials for such purposes, see Romauldi U.S. Pat. No. 3,429,094.
Also noted as of interest are Graham U.S. Pat. Nos. 865,490 and 983,274; Henderson U.S. Pat. No. 1,891,763; Rubenstein U.S. Pat. No. 2,850,890; Naaman U.S. Pat. No. 3,852,930; Schupack U.S. Pat. No. 4,159,361; and Matsumoto U.S. Pat. No. 4,379,870; as well as U.K. Patent No. 578,036; Japanese Patent No. 2,141,206; and German Patent No. 3,342,626. Of these, Graham U.S. Pat. Nos. 865,490 and 983,274 disclose a reinforced concrete slab which is designed and intended for placement on the ground. These references includes reinforcing rods in the bottom half, with the latter of these references including the addition of what appears to be a high volume of short wire sections in the upper portion of the concrete to increase the strength of the slab. Because of the size and volume of the wire sections they are added by placing them on top of the concrete and allowing them to settle into the concrete. Graham neither teaches nor suggests a load bearing panel intended to be placed on two or more spaced apart supports, and in the more than eighty years since its filing, its application to load bearing panel construction technology is not known to have occurred. Schupack U.S. Pat. No. 4,159,361 discloses cold formable, reinforced panel structures which include shrinkage and thermal reinforcement fibers. Schupack neither teaches nor suggests a load bearing panel which is intended to be placed on two or more spaced apart supports, nor a panel which includes flexural reinforcing material, and its application to load bearing panel construction technology is neither taught nor suggested. Matsumoto U.S. Pat. No. 4,379,870 discloses a specific form of synthetic resin reinforcement material which has utility in concrete structures, but it neither teaches nor suggests a load bearing panel which is intended to be placed on two or more spaced apart supports, nor a panel which includes flexural reinforcing material, and its application to load bearing panel construction technology is neither taught or suggested.
It is important to here note that "reinforcement material" as used throughout this application is different from "flexural reinforcing material," such as traditional steel re-bars.